1. Field of the Invention
The present invention relates to an electron-emitting device, and more particularly to an electron-emitting device having the structure in which an insulating layer is interposed between a pair of electrodes. It also relates to an electron beam lithography machine and an image display apparatus which make use of the device.
2. Related Background Art
FIGS. 6 and 7 diagramatically illustrate a constitution commonly taken in MIM-type electron-emitting devices.
As shown in FIGS. 6 and 7, an electron-emitting device of the MIM-type comprises an electrode 1 and a surface electrode 3 which is thin and laminated thereon, and interposing therebetween an insulating layer 2 which is thin. The electrode on the side from which electrons are emitted is hereinafter particularly called the surface electrode. Then, a voltage V larger than the work function .phi.m of a metal used in the surface electrode 3 is applied between the electrode 1 and the surface electrode 3. As a result, among electrons that have tunneled through the insulating layer 2, those having a larger energy than the vacuum level are emitted from the surface of the surface electrode 3.
In order to obtain a high electron-emission efficiency in such a device, it is preferable that the insulating layer 2 is made small in thickness so that the energy of tunneled electrons and the number thereof can be increased and also that as shown in FIG. 7 the surface electrode 3 should preferably be formed with a thickness as small as possible so that a decrease in energy because of the scattering or the like in the surface electrode 3 is prevented. It is further preferable that a metallic material having a small work function .phi.m is used in the surface electrode 3 (Japanese Unexamined Patent Publications No. 124327/1988 and No. 141234/1988).
However, in the case when a film thickness is very small, metals commonly tend to take an island structure, and it is very difficult to form such a metallic thin film as mentioned above. Moreover, even if it has become possible to form a metallic thin film as the surface electrode, since the resistance is larger, particularly there is a high specific resistance at metal materials having a small work function, a fall of voltage in the surface electrode 3 can not be ignored and thus there is a possibility that a uniform and effective electric field is not applied to the electron-emitting device. There is also a possibility that a loss of electric power in the surface electrode 3 because of such a fall of voltage causes a break in the device. Hence, there is a limit to the formation of the surface electrode 3 with a small thickness, resultingly also bringing about a limit to the improvement in electron-emission efficiency.
The electron-emission efficiency can be improved to a certain extent with a decrease in thickness of the surface electrode, but, on the other hand, new problems may arise such that the cross-sectional shape of an electron beam emitted from the surface electrode 3 becomes poor and also the fluorescent luminance attributable to the electron beam also becomes non-uniform with a decrease in thickness of the surface electrode 3. More specifically, when a fluorescent member capable of exhibiting fluorescence as a result of irradiation with electron beams is disposed right above the surface electrode 3, it follows, with a decrease in thickness of the surface electrode 3 as in the above, that a fluorescent spot formed on the surface of the fluorescent member does not reflect the electrode shape of the surface electrode 3 to have an unnecessary spread. In addition, the luminance at the above fluorescent spot may also become non-uniform, resulting in a fluorescence uneveness. Moreover, once the cross-sectional shape of the electron beam has become bad or the fluorescent luminance has become non-uniform (luminance uneveness) as mentioned above, there must arise the problem that the resolution of an image is lowered, the luminance is lowered and the luminance uneveness occurs particularly when such an MIM-type electron-emitting device is used as an electron source of an image display apparatus or an electron beam lithography machine.